How It Works
displays are usually configured with a screen-size variable. That is, how big your screen is in real-world measurements, not pixels. grabs this value and uses it come up with a (dots per inch) value for your display, and thus can relate the number of pixels you've traversed to the number of inches the pointer appears to have moved on the screen.
But lets be real! Its horribly inaccurate! makes no pretense of being even remotely accurate. In fact, someone has measured the movement of their mouse across the screen with a ruler, and found that the distance algorithm is almost 25% off! The original author's response to this complaint was who cares?.